The increasing scarcity of raw materials for energy production has created a growing need worldwide to produce energy from renewable resources. Due to its continuous availability, regardless of short-term fluctuations in wind or sunlight, bioenergy, and thus also biogas, makes a worthwhile contribution as an addition to the energy mix of renewable energy sources.
The production of thermally useable gas mixtures from biodegradable substrates (biogas extraction) represents a technically relatively easy process for converting biomass into energy. The biochemical decomposition of organic substances by microorganisms under anaerobic conditions (fermentation) is well known.
Prior to processing, biogas itself is a mostly water-saturated combustible gas mixture comprising the main components methane (CH4) and carbon dioxide (CO2). It also contains small quantities of nitrogen (N2), oxygen (O2), hydrogen sulphide (H2S), hydrogen (H2) and ammonia (NH3). The methane content is the most important in terms of the utilization of biogas since, as said methane is an oxidisable compound, it releases energy during combustion.
To increase the economic efficiency of biogas production, general efforts are made to convert the biomass used as fully as possible. Maximising the volumetric loading (mass of dry organic substance added per unit of fermenter volume and time) is also desirable. At the same time, substrates that are as energy-rich as possible are used.
However, volumetric loadings that are too high can also easily lead to an imbalance in the multi-stage fermentation process. Acidification in the fermenter caused by this leads to the inhibition of methanogenic microorganisms. In extreme cases, this can lead to a complete disruption of the process. In order to offset acidification processes of this kind, it is customary to reduce the substrate supply where required. If this course of action is unsuccessful, a complete replacement of the reactor content is required. The subsequent starting of the fermentation process until maximum gas yield has been achieved can take up to twelve weeks. The reduction in methane production or the complete disruption with subsequent start-up results directly in reduced profits.
Another common method for process stabilisation consists in countering substrate acidification by adding neutralising additives. Lime-based products have proven particularly suitable for this purpose. JP 632708 describes the anaerobic fermentative treatment of a mixture of liquid soya bean residue and aluminium oxide, wherein a combination of zeolites and burnt lime (CaO) is added to enhance fermentation.
The use of lime-based products to stabilise biogas production is also known from DE 100 34 279 A1. This document describes a process for controlling a biogas plant in which parameters of the silage effluent, for example, pH value, which are important in terms of methanation, are measured using a measuring device. It discloses that hydrated lime or lime milk is added if the pH falls below a specific value, and consequently the pH value rises to the required level again. EP 2 090 660 A1 describes a method for producing biogas by adding carbonated lime.
The disadvantage of the known methods is that the lime-based products added tend to settle in the bioreactor which leads to a reduction in their effectiveness.